摘要:

In most recent free oscillation studies of the earth's interior the effect of absorption upon the eigenperiods of the earth has been ignored. This is equivalent to assuming that the earth is close to perfectly elastic. Since the actual earth is significantly anelastic over seismic frequencies, a frequency‐dependent correction of the order of 1% must be applied to the normal mode periods in order that models based on those modes may be compared with body wave results. The eigenperiods of an earth model based on uncorrected data, model C2 (Anderson and Hart, 1976a), are adjusted for attenuation and then inverted to determine the resulting overall perturbation in the earth model. The corrected normal mode data as well as the uncorrected data can be fitted by a spherically symmetric earth model. The effect of including the attenuation term is to generally increase seismic velocities, particularly shear velocities, throughout the model. An important consequence of this change is to reduce or eliminate base line discrepancies between body wave results and normal mode result

ISSN:0148-0227    

DOI:10.1029/JB082i011p01647

年代:1977

数据来源: WILEY

摘要:

Creep meters spanning an active trace of the San Andreas fault in central California recorded many apparent fault‐slip steps at times of moderate local earthquakes of magnitudes 4–5. These steps are small, (less than 1 mm) but are recorded over long fault segments (tens of kilometers). The corresponding strain steps, ranging from 10−5to 10−4, are about 2 orders of magnitude larger than those calculated from the seismically determined source parameters on the assumption of uniform crustal materials. These anomalously large coseismic steps may be due to low effective rigidity of the fault gouge zone or to seismically triggered secondary surface breaks that occurred along th

ISSN:0148-0227    

DOI:10.1029/JB082i011p01655

年代:1977

数据来源: WILEY

摘要:

The dynamical earthquake model of Burridge and Halliday (1971) is embedded in a completely relaxing viscoelastic solid half space with high viscosity. In this way, Budiansky and Amazigo's (1976) long‐term creep mechanism for earthquake repetition is combined with a tractable dynamical model of the earthquakes themselves to complete a picture of long‐term stick‐slip on a seismic fault. A simple periodic solution is obtained in which all earthquakes are identical. The stability of this periodic behavior and the questions of whether and how this state is approached from an arbitrary initial state are left unres

ISSN:0148-0227    

DOI:10.1029/JB082i011p01663

年代:1977

数据来源: WILEY

摘要:

Comparison of preearthquake vertical geodetic surveys with postearthquake surveys in the region affected by the magnitude 5.7 Oroville earthquake of August 1, 1975, indicates appreciable elevation changes. The data are consistent with 0.36 m of normal slip on a 12×10 km2rectangular fault dipping 60° to the west. Comparison of level surveys run 1 and 6 months after the earthquake suggests an additional 0.08 m of postseismic slip on the fault. Horizontal geodetic data do not show any significant changes, but the horizontal data are noisier than the vertical data and allow as much as 0.7 m of coseismic left lateral slip within a 95% confidence limit. There is no evidence for postearthquake dilatancy recovery, but the data are not adequate to exclude small amounts (10 mm) of relaxation, or even larger amounts if the dilatant region were very broad (80 km in diameter

ISSN:0148-0227    

DOI:10.1029/JB082i011p01667

年代:1977

数据来源: WILEY

摘要:

Thermal inertia of the earth's surface can be used in geologic mapping as a complement to surface reflectance data as provided by Landsat. Thermal inertia cannot be determined directly but must be inferred from radiation temperature measurements (by thermal IR sensors) made at various times in the diurnal cycle, combined with a model of the surface heating processes. We have developed a model which differs from models created previously for this purpose, because it includes sensible and latent heating. Tests of this model using field data indicate that it accurately determines the surface heating. When the model is used with field measurements of meteorological variables and is combined with remotely sensed temperature data, a thermal inertia image can be produced.

ISSN:0148-0227    

DOI:10.1029/JB082i011p01673

年代:1977

数据来源: WILEY

摘要:

The amount of smooth plains material in craters surrounding the Caloris basin on Mercury depends on the extent these craters have been eroded by Caloris ejecta. Therefore smooth plains surrounding Caloris must have been emplaced at least in part by saturated secondary cratering. Mercurian uplands outside the continuous belt of smooth plains deposits have a crater population which is deficient in craters smaller than 50 km relative to extrapolations of craters larger than 50 km and relative to size frequency distributions of craters ≤50 km on typical lunar upland regions. However, the typical lunar upland regions have been masked by the addition of numerous large basin secondary craters. Only rare areas southwest of Mare Serenitatis appear to be similar to Mercurian terrain at great distances from smooth plains because the areas are relatively free of basin secondaries. Martian uplands also exhibit a crater population which is deficient in craters less than 30–50 km, which was previously interpreted to have been caused by obliteration of some craters less than 30 km by surface processes. The observed crater deficiencies on the moon, Mars, and Mercury below 30–50 km are mostly a reflection of the primary crater production population; it is characterized by a power function having two different exponents (−2.0 for craters less than 50 km and −3.2 to 3.5 for craters ≥50 km). The hypothesis that the observed deficiency of small lunar craters southwest of Mare Serenitatis resulted from an episode of premare volcanism which obliterated some small craters was evaluated and rejected because highland volcanic rocks are rare in returned samples, and if significant premare upland basaltic volcanism is hypothesized, it would be circumstantial that it occurred only in those areas least affected by basin ejecta. Premare upland volcanic activity would be required also to explain the Mercurian crater counts. However, the Mercurian mapping results indicate that intercrater plains representative of hypothetical volcanic rocks were formed before craters now observed on their surfaces. The most persuasive reason for rejecting the theory of lunar premare volcanism is that reexamination of all available crater count data for lunar uplands shows no evidence for a primary production population characterized by a single power function. Therefore crater counts showing deficiencies relative to a single power function which previously was acceptable evidence for crater obliteration need no longer be considered as evidence for crater o

ISSN:0148-0227    

DOI:10.1029/JB082i011p01681

年代:1977

数据来源: WILEY

摘要:

The lunar gravitational research reported on by Gapcynski et al. (1975) has been extended to include an additional 600 days of the time variation of ascending node for the Explorer 49 spacecraft. Analysis of these additional data resulted in an improved value of the second‐degree zonal harmonic coefficientC20= (−2.0219±0.0091) × 10−4. This value ofC20used in conjunction with β = (631.27±0.03) × 10−6and γ = (227.7±0.7) × 10−6yields a more accurate definition of the lunar moment of inertia ratio ofC

ISSN:0148-0227    

DOI:10.1029/JB082i011p01699

年代:1977

数据来源: WILEY

摘要:

Significant changes in the reversal frequency of the geomagnetic field seem to occur typically at time intervals of the order of 30–50 m.y. It appears unlikely that these long‐term variations in the behavior of the magnetic field would arise through fluctuations in a geomagnetic dynamo operating under uniform conditions. Instead, they may reflect changes in the conditions in the core caused by fluctuations in the temperature at the core‐mantle boundary. If the geomagnetic dynamo is maintained by thermal convection in the outer core, convection in the mantle may be required to transport the heat that crosses the core‐mantle boundary through the mantle. Numerical investigations of thermal convection at high Rayleigh numbers admit the possibility that this convection will be intermittent, consisting of rising plumes of material, which are occasionally emitted from a thermal boundary layer at the base of the mantle. During the convective part of the intermittent cycle, temperatures at the core‐mantle boundary decrease, leading to an increase in the Rayleigh number of the core. When the lower mantle again becomes quiescent, temperatures at the core‐mantle boundary rise and the Rayleigh number of the core decreases. These fluctuations in the Rayleigh number of the core are likely to disturb the flow field there and may affect the frequency of geomagnetic reversals through the mechanism suggested by Levy. The seismic properties ofD″ can be explained by a thermal boundary, layer at the base of the mantle, and periods of intermittent convection in the mantle calculated from Foster's analysis are reasonably consistent with the long‐term trends in reversal frequency inferred from the geo

ISSN:0148-0227    

DOI:10.1029/JB082i011p01703

年代:1977

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/JB082i011p01710

年代:1977

数据来源: WILEY